Vector force-balancing mechanisms

ABSTRACT

A vector force-balancing mechanism comprising a first rigid member connected to a frame for receiving an input force to be balanced, a second rigid member connected to the first member by a flexure strip for relative pivotal movement, and a third rigid member lockably pivotally mounted on the frame about an axis which is perpendicular to the plane of the relative movement and extends transversely through the flexure strip when the first and second rigid members are relatively angularly positioned so that the flexure strip is unstrained, the third rigid member being pivotally attached to the second member at a position remote from the axis.

United States Patent [1 1 Boden VECTOR FORCE-BALANCING MECHANISMS [75]Inventor: Peter Standidge Boden, Harpenden,

1 England [73] Assignee: Kent Instruments Limited, Luton,

Bedfordshire, England [22] Filed: Sept. 24, 1971 [21] Appl. No.: 183,353

[30] Foreign Application Priority Data [56] References Cited UNITEDSTATES PATENTS Gorrie 74/469 X fi ms rzlewss J- Gill A ttorney irvin S.Thompson andRobert .i Patch [57] ABSTRACT A vector force-balancingmechanism comprising a first rigid member connected to a frame forreceiving an input force to be balanced, a second rigid member connectedto the first member by a flexure strip for relative pivotal movement,and a third rigid member lockably pivotally mounted on the frame aboutan axis which is perpendicular to the plane of the relative movement andextends transversely through the flexure strip when the first and secondrigid members are relatively angularly positioned so that the flexurestrip is unstrained, the third rigid member being pivotally attached tothe second member at a position remote from the axis. J

2 Claims, 4 Dravving Figures PAIENIEU JW 9 3 MUIOFZ 1 VECTOR FORCE-BALANCING MECHANISMS SUMMARY OF THE INVENTION This inventionrelates to vector force-balancing mechanisms as used, for example, inpressure monitoring instruments.

According to the present invention, a vector forcebalancing mechanismcomprises a frame, a first rigid member connected to said frame forreceiving an input force to be balanced, a second rigid member, aflexure strip connecting together said two rigid members for relativepivotal movement, and a third rigid member lockably pivotally mounted onsaid frame about an axis which is perpendicular to the plane of saidrelative movement and extends transversely through said flexure stripwhen said first and second rigid members are relatively angularlypositionedso that said flexure strip is unstrained, said third rigidmember being pivotally attached to the second member at a positionremote from said axis.

Varying of the relative positioning of the first and second memberswithin a desired range to vary the ratio between the applied force and abalancing force is carried out by unlocking the third rigid member androtating it to move the second member with respect to the first. Thethird member can then be locked when the relative positioning of thefirst and second members, i.e., the angle therebetween, is set to a newvalue.

With the mechanism of the invention, as is described in more detailbelow, the nature of the relative movement of the first and second rigidmembers approaches that which would be obtained if the members wereconnected by a hinge, i.e., there is a minimum of translational relativemovement associated with the relative rotational movement and movementof the first rigid member with respect to the fixed reference inresponse to relative rotation of the first and second members is thusminimal. The ratio between the length of the flexure strip as measuredin the plane of said relative movement, between the positions where itmeets the first and second rigid members, and the spacing between thepoint where the flexure strip meets the first member and said axis whenthe flexure strip is unstrained, as measured in like manner, affects theextent of said minimal movement of the first member with respect to theframe of reference when the two members are relatively displaced. It hasbeen found when such ratio is given the value 321 that such movement isminimized.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERREDEMBODIMENT A vector mechanism for balancing an applied force part of themechanism 3 on an enlarged F and a balancing force F so that F, shown inFIG. 1. The mechanism comprises a first rigid member or main beam 12 anda second rigid member or elevating beam 14, the two members beingpivotally connected together at 13 about an axis normal to the plane ofthe drawing and shown as O. The elevating beam 14 is pivotally connectedat 16 to a fixed frame shown schematically as R. The main beam 12 ispivotally connected at 18 to the frame R via a member 20 and a furtherpivotal connection 22, and is also connected to the frame R via members24, 26 and further pivotal connections 28,30.

The vector mechanism is arranged so that the angle 6 between the mainbeam 12 and the elevating beam 14 may be varied over a given range sothat the ratio between the balancing force F and the applied force F maybe correspondingly varied. It is necessary that the members 20, 24 and26 do not move as 0 is varied, i.e., that the main beam 12 does not moveby more than a negligible amount. This can be catered for by arrangingfor the pivotal connection 16 to be adjustable with respect to the frameR over the arc of a circle centered at O.

A preferred embodiment of the present invention is shown in FIGS. 2, 3and 4. In these figures, where ap' propriate, the reference numeralsused are the same as those used for corresponding elements shownschematically in FIG. 1. The pivotal interconnections 13, 16, 18 areflexure strips which may be made of thin, elastic metal. Flexure stripsare used because hinge joints are generally unsatisfactory in very lightmechanisms due to the inaccuracies caused by hysterisis and friction. Toaccomplish the required connection via pivotal interconnection 16 to theframe R, the interconnection 16 is made to a third rigid member, aso-called locking arm 32, which is lockably pivotally mounted to theframe R about an axis X-X which is normal to the plane of relativemovement of the beams 12 and 14. To adjust the angle 0, the locking arm32 is unlocked, the beams 12 and 14 are pivoted to a desired relativeposition and the arm 32 is then locked again.

As mentioned above, it is important that the main beam 12 does not moveby more: than a negligible amount when the angle 6 is varied. To ensurethis, it will be apparent that the axis X-X of the locking bar 32 mustcoincide as far as possible with the axis 0 of the pivotalinterconnection 13. However, with a flexure strip, there is nowell-defined pivotal axis. Nevertheless, movement of the main beamduring adjustment of the angle 0 is minimized in the present inventionby arranging for the axis X-X to extend transversely through the flexurestrip 13 when it is unstrained. This may readily be seen from FIG. 3where the unstrained position of the flexure strip 13 and of theelevating beam 14 are shown in dotted lines.

Movement of the main beam 12 can be further minimized by suitablychoosing the ratio between the length of the flexure strip 13 asmeasured between the points where it meets the main beam 12 and theelevating beam M, i.e., the dimension y in FIG. 4, and the spacingbetween the point where the flexure strip 13 meets the main beam 12 andthe axis X-X,. when the strip 13 is unstrained, i.e., the dimension x inFIG. 4. It has been found that movement of the main beam is minimal whenthis ratio is 3:1. With such an arrangement, in a vector mechanism wherethe dimension y of the strip 13 was about 0.25 inches, the main beammovement F, tan 0 is was observed to be only about one ten thousandth ofan inch (0.0001 inches) for a change in 9 of 30.

The invention may be embodied in other forms than that shown in FIGS. 2,3 and 4. For example, the unstrained position of the flexure strip 13 isdesirably arranged to occur at the mean value of 6, which may be otherthan zero as in the embodiment shown here.

It will thus be seen from the foregoing disclosure that the presentinvention provides a vector mechanism which employs flexure stripswhilst minimizing the attendant disadvantage while main beam movement.

I claim:

1. A vector force balancing mechanism comprising a first rigid memberadapted to be connected to a frame for receiving an input force to bebalanced, a second rigid member, a flexure strip connecting togethersaid two rigid members for relative pivotal movement, and

a third rigid member adapted to be lockably pivotally mounted on saidframe about an axis which is perpendicular to the plane of said relativemovement and extends transversely through said flexure strip when saidfirst and second rigid members are relatively angularly positioned sothat said flexure strip is unstrained, said third rigid member beingpivotally attached to the second member at a position remote from saidaxis.

2. A vector force-balancing mechanism as claimed in claim 1, wherein theratio between the length of the flexure strip between the points whereit meets said first and second rigid members and the spacing between thepositions where said flexure strip meets said first rigid member andwhere said strip is intersected by said axis, each measured in saidplane of relative movement, is 3:1.

1. A vector force balancing mechanism comprising a first rigid member adapted to be connected to a frame for receiving an input force to be balanced, a second rigid member, a flexure strip connecting together said two rigid members for relative pivotal movement, and a third rigid member adapted to be lockably pivotally mounted on said frame about an axis which is perpendicular to the plane of said relative movement and extends transversely through said flexure strip when said first and second rigid members are relatively angularly positioned so that said flexure strip is unstrained, said third rigid member being pivotally attached to the second member at a position remote from said axis.
 2. A vector force-balancing mechanism as claimed in claim 1, wherein the ratio between the length of the flexure strip between the points where it meets said first and second rigid members and the spacing between the positions where said flexure strip meets said first rigid member and where said strip is intersected by said axis, each measured in said plane of relative movement, is 3:1. 